Fix #1665 missing images for editor buttons

Added icons for video/PHP buttons in Buttons editor theme, changed the
duplicate code button in Monocons theme to Ⓟ character (couldn't find
any more suitable chars).

https://github.com/mybb/mybb/issues/1665
Merge pull request #1699 from Destroy666x/fix1665

Fix #1665 missing images for editor buttons